Chemical compounds are typically mixed within an agitator tank containing a rotating impeller. Bottom-mounted magnetically-coupled mixers can be used for such applications. Such mixers typically include an agitator tank having within it an impeller assembly comprised of a plurality of flat, rectangular radial-pumping blades. The agitator tanks also include a magnetic coupler which couples the impeller to a motor located at the exterior of the tank. Magnetic couplers typically contain a drive magnet attached to the motor and a driven magnet connected to the impeller. The magnets are placed in close proximity to each other so that rotation of the drive magnet induces a rotation of the driven magnet impeller. The impeller assembly operates in close proximity to the tank bottom. As a consequence of the impeller's proximity to the bottom of the tank and the use of radial pumping devices, bottom-mounted magnetically-coupled mixers are typically only used in connection with small vessels for minor mixing duty. Examples of such bottom-mounted magnetically-couple mixers can be found in U.S. Pat. Nos. 5,364,184, 5,378,062, 5,470,152, 5,758,965, and 5,779,359.
Rotating, levitating magnetic elements which levitate above a cold magnetized superconducting material can also be used to mix or pump fluids. Similar to bottom-mounted magnetically coupled mixers, levitating mixers include an agitator tank having within it an impeller assembly comprised of a plurality of blades, and a magnetic coupler which couples the impeller to a motor located at the exterior of the tank. Unlike bottom-mounted magnetically-coupled mixers, superconducting levitation technology allows the impeller assembly to “float” inside the tank without any mechanical support. Rather, the supporting force comes from superconducting material that “freezes” a magnetic field within which an impeller can be rotated but not moved horizontally or vertically (within the limits of the magnetic field). An example of such a device can be found in U.S. Pat. No. 5,416,215.
A drawback associated with current superconducting levitation mixers is that by nature of the fluid flow, the local pressure above the impeller is lower than the pressure below the impeller, thus producing a net upward force underneath the impeller. This force can become sufficiently strong to override the magnetic attraction to the superconducting magnets on the outside of, the vessel, leading to decoupling of the impeller.
Accordingly, it is desirable to provide a method and magnetically-coupled impeller apparatus for mixing or pumping fluids that is suitable for use beyond the small vessels and minor mixing duty typically associated with bottom-mounted magnetically-coupled mixers. It is also desirable to provide a method and magnetically-coupled impeller apparatus that minimizes impeller instability in superconducting levitation mixers.